There are two types of tennis balls, namely pressurized and non-pressurized. Pressurized tennis balls have hollow cores that are filled with what gas?

Pressurized tennis balls are filled with nitrogen gas (or sometimes air, which is mostly nitrogen) to maintain consistent bounce and feel. The internal pressure, typically around two atmospheres, helps the ball retain its performance characteristics during play. Oxygen would cause the rubber to degrade faster, while hydrogen is too reactive and carbon dioxide doesn't maintain pressure as effectively.